Prior to 1969, the aircraft industry paid little concern to commercial jet aircraft engine noise. In December 1969, the U.S. Federal Aviation Administration ("FAA") promulgated specific noise level regulations for aircraft under authority of Public Law 90-411. Existing airplanes were required to be certificated for compliance with Federal Air Regulation No. 36 ("FAR 36"). Similar noise standards were prescribed by international civil aviation organizations (for example, "ICAO Annex 16"). Several states and municipalities also established airport noise levels. Thus, it became imperative for airframe and engine manufacturers and owners to take noise considerations into account in designing, building and using jet aircraft.
By December 1970, Douglas Aircraft Co., on behalf of the National Aeronautics and Space Administration, had undertaken studies concerning fan-compressor noise from the Pratt & Whitney JT3D-3B engines used with DC-8-50/61 airplanes made by Douglas. The studies showed that noise levels might be reduced in a short duct nacelle design having revised fan inlet and exhaust ducts containing acoustically absorptive linings. Various configurations were tested. Although it was shown that typically up to 10.5 EPNdB ("effective perceived noise") reduction in the noise level could be achieved (at maximum certified landing weight and at a point on the ground beneath a 3.degree. landing-approach path one nautical mile from the runway threshold), static take-off-rated gross thrust was reduced by 2.5% and fuel consumption was increased by 3%. Moreover, direct operating costs were estimated to increase 4.4% and return on investment would decline about 6%.
At the same time, The Boeing Co. was conducting similar studies of noise reduction of JT3D engines used with Boeing 707-320C airplanes. The studies showed that noise reductions up to 15 EPNdB could be achieved on landing approach. It was planned to accomplish this reduced noise level by installing two acoustically treated rings in the engine inlet and by acoustically treating an extended 3/4 length fan duct configuration. That modification resulted in a range reduction of 200 nautical miles and a direct operating cost increase of more than 9%. Further, the use of rings may interfere with de-icing of the engines.
Thereafter, the major airframe manufacturers undertook substantial research effort toward developing retrofit kits for existing airplanes to attempt to meet FAR 36 noise level requirements. However, they were unable to develop kits which would meet the noise requirements without, at the same time, degrading performance, increasing fuel consumption and unreasonably increasing costs. As a result of the unavailability of retrofit hush kits, the value of existing airplanes fell significantly as their useful lives neared an end. As of Oct. 1, 1979, there were about 155 DC-8 airplanes with JT3D engines in service by U.S. airlines and about 218 in service by foreign airlines. Although the effective date of the FAR 36 noise requirements was extended on several occasions, and the requirements were modified, the regulation finally became effective as of Jan. 1, 1985. On that date, the existing DC-8 airplanes became essentially obsolete for use in the United States.
The studies which had been conducted showed that noise radiates from a low by-pass, fan jet engine in several directions. High-frequency fan noise radiates both forward through the air inlet cowl and aftward through the exhaust ducts. Low-frequency jet noise generally radiates rearwardly. At low engine thrust, the high-pitch whine of the fan is more pronounced. At high engine thrust, the low-pitch jet rumble is more noticeable.
Each noise component must be dealt with separately, as well as in combination. Generally, some studies showed that noise attenuation material was useful for reducing some noise components. One type of material frequently used consists of honeycomb core cells bonded to a porous sheet on the airflow surface and an impervious sheet on the rearward surface. Such noise attenuation material has been used in several different nacelle configurations for different airplanes.
Other studies showed that nacelle modifications could suppress engine noise. However, these modifications which, it appeared, could succeed in suppressing noise would also severely degrade airplane performance or substantially increase fuel consumption.
Thus, although existing DC-8 airplanes could not be used, at least in the United States, after implementation of the FAR 36 noise level requirements, no one had developed and certified with the FAA a system to reduce the noise levels and thereby make the obsolete DC-8 airplanes economically viable.